Small ribonucleoprotein particles (snoRNPs) are protein-RNA assemblies that catalyze specific chemical modifications on ribosomal RNA, small nuclear RNA, transfer RNA and some messenger RNA. A common modification required for ribosomal RNA biogenesis is 2'-O-methylation that is catalyzed by box C/D snoRNPs. Currently very little is known about the molecular mechanism of assembly and catalysis of box C/D snoRNPs. A multidisciplinary approach will be undertaken to study the assembly and catalytic function of box C/D snoRNPs that employs crystallographic, biochemical, and mass spectroscopic methods. The following specific aims will be achieved during the proposed period: 1) understand protein-protein interactions required for snoRNP assembly both by determining a crystal structure of the protein complex of two core snoRNP proteins and by the hydrogen/deuterium exchange mass spectroscopy; 2) identify and characterize the catalytic subunit(s) of the box C/D snoRNP by crystallographic as well as biophysical binding studies on binding of the predicted methyl donor, S-adenosyI-L-methionine; 3) understand the interaction between the snoRNP proteins and the box C/D RNA required for assembly by biophysical binding studies, co-crystal structure determination, as well as mutagenesis studies between each protein and the box C/D RNA; and 4) obtain a crystal structure of the intact snoRNP including a methylation RNA target. This study will make significant contributions to our understanding of ribosome biogenesis and gene regulation. It also has direct implications in understanding the molecular mechanisms for several diseases linked to the function of box C/D snoRNPs.